Studies will be continued to examine the mechanism responsible for the renal secretion of potassium. By comparing animals in which selected areas of the kidney are destroyed it will be possible to determine whether cortical or medullary portions of the distal tubule are of predominant importance in whole kidney potassium secretion. In parallel studies micropuncture experiments will be conducted in order to determine the relative importance of the distal convolution and collecting duct in potassium secretion, under conditions of uremia. In addition, further studies will be conducted in a newly established model, which utilizes the rectal gland of the dogfish shark, Squalus acanthias, to examine the role of carbonic anhydrase in chloride transport.